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What's Blood?

Reviewed by: Larissa Hirsch, MD

You know what blood is — it's that red stuff that oozes out if you get a paper cut. But what is blood, really, and what does it do?

What Is Blood and What Does It Do?


Blood is made up of blood cells and plasma. Plasma (say: PLAZ-muh) is a yellowish fluid that has nutrients, proteins, hormones, and waste products. The different types of blood cells have different jobs.

What Are the Types of Blood Cells?

Red blood cells: Red blood cells (RBCs, also called erythrocytes; say: ih-RITH-ruh-sytes) are shaped like slightly indented, flattened disks. RBCs contain hemoglobin (say: HEE-muh-glow-bin), a protein that carries oxygen. Blood gets its bright red color when hemoglobin picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the different body parts.


White blood cells: White blood cells (WBCs, also called leukocytes, say: LOO-kuh-sytes) are a key part of the immune system. The immune system helps the body defend itself against infection. Different types of WBCs fight germs, such as bacteria and viruses. Some types of WBCs make antibodies, which are special proteins that recognize foreign materials and help the body get rid of them.





How Does Blood Travel in the Body?



Two types of blood vessels carry blood throughout our bodies:



What if Someone Has Low Numbers of Blood Cells?

Sometimes medicine can be given to help a person make more blood cells. And sometimes blood cells and some of the special proteins blood contains can be replaced by giving a person blood from someone else. This is called a transfusion (say: trans-FEW-zyun).


Hey, What's Your Type?


Besides getting a letter or two, a person's blood is either "positive" or "negative." This is a way of keeping track of whether someone's blood has a protein called Rh protein. If your blood is positive, you have this protein. If it's negative, you don't. Either way is totally fine.

People have one of these eight different blood types:

  1. A negative
  2. A positive
  3. B negative
  4. B positive
  5. O negative
  6. O positive
  7. AB negative
  8. AB positive
Reviewed by: Larissa Hirsch, MD
Date reviewed: January 2019